Abstract : In describing the social ascension of a group of valiant knights by their competence alone, the "Poem of the Cid" proclaims the model of a personal seigniory intended to correct the ill effects of a regime founded essentially on 'nature'. On the other hand, alphonsine historiography, destined to serve the centralistic aspirations of the Crown, attemps to impose the model of a royal order. In spite of the fact that these two schools of thought differ as much by their form as by the values they put forward, the "Poem of the Cid" constitues the essentiel narrative material of the "History of Spain" for the history of Rodrigue Diaz and for that of Alphonse the VIth. In these conditions, what procedures are employed to transfer the epic discourse into an historiographic one? A meticulous study of the similarities between the versions of the "History of Spain" ("Chronicle of twenty kings", "Sancian version", "Chronicle of Castille") allows the distinction of the two criteria used to select narrative information: its plausibility and its functionality. Four operations express the mechanisms by which the epic is integrated into history: segmentation, uniformisation, abbreviation, and, most importantly, amplification. On these alterations depens the essential part of the monarchist purpose: to accomplish judicial centralisation and to encourage the exemplary act of dependance by the subject before his natural lord. On another level, the analysis of the differences between the texts informs us of the evolution of historiographic discourse itself. Although the aim of historiography remains unchanged, the voice of its authors is expressed in a more personal way. In this way, the text gives a large place to the chivalrous nobility, those men who desire to climb the state hierarchy by entering the services of the king and who, in the last third of fourteenth century, formed the great Houses of Trastamore.